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[PATCH v7 2/4] Use int64 to represent instr_time on all platforms
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* [PATCH v7 2/4] Use int64 to represent instr_time on all platforms
@ 2023-01-17 04:38 Andres Freund <andres@anarazel.de>
  0 siblings, 0 replies; 371+ messages in thread

From: Andres Freund @ 2023-01-17 04:38 UTC (permalink / raw)

Until now we used struct timespec on all platforms but windows. That causes a
fair bit of memory (struct timeval is 16 bytes) and runtime overhead (much
more complicated additions). Instead we can convert the time to nanoseconds in
INSTR_TIME_SET_CURRENT(), making the remaining operations cheaper.

Representing time as int64 nanoseconds provides sufficient range, ~292 years
relative to a starting point (depending on clock source, relative to the unix
epoch or the system's boot time). That'd not be sufficient for calendar time
stored on disk, but is plenty for runtime interval time measurement.

On windows instr_time already is represented as cycles. It might make sense to
represent time as cycles on other platforms as well, as using cycle
acquisition instructions like rdtsc directly can reduce the overhead of time
acquisition substantially. This could be done in a fairly localized manner as
the code stands after this commit.

Because the windows and non-windows paths are now more similar, use a common
set of macros. To make that possible, most of the use of LARGE_INTEGER had to
be removed, which looks nicer anyway.

Author: Andres Freund <andres@anarazel.de>
Author: Lukas Fittl <lukas@fittl.com>
Author: David Geier <geidav.pg@gmail.com>
Reviewed-by:
Discussion: https://postgr.es/m/20230113195547.k4nlrmawpijqwlsa@awork3.anarazel.de
---
 src/include/portability/instr_time.h | 149 +++++++++++++--------------
 1 file changed, 73 insertions(+), 76 deletions(-)

diff --git a/src/include/portability/instr_time.h b/src/include/portability/instr_time.h
index 9ea1a68bd94..185be8d5cad 100644
--- a/src/include/portability/instr_time.h
+++ b/src/include/portability/instr_time.h
@@ -34,6 +34,8 @@
  *
  * INSTR_TIME_GET_MICROSEC(t)		convert t to uint64 (in microseconds)
  *
+ * INSTR_TIME_GET_NANOSEC(t)		convert t to uint64 (in nanoseconds)
+ *
  * Note that INSTR_TIME_SUBTRACT and INSTR_TIME_ACCUM_DIFF convert
  * absolute times to intervals.  The INSTR_TIME_GET_xxx operations are
  * only useful on intervals.
@@ -54,8 +56,26 @@
 #ifndef INSTR_TIME_H
 #define INSTR_TIME_H
 
+
+/*
+ * On all platforms we store time using a 64bit integer - they're cheap to
+ * add/subtract, the most common operations. The acquisition of time
+ * and converting to specific units of time is platform specific.
+ */
+
+typedef int64 instr_time;
+
+
+/* helpers macros used in platform specific code below */
+
+#define NS_PER_S	INT64CONST(1000000000)
+#define NS_PER_MS	INT64CONST(1000000)
+#define NS_PER_US	INT64CONST(1000)
+
+
 #ifndef WIN32
 
+
 /* Use clock_gettime() */
 
 #include <time.h>
@@ -80,93 +100,39 @@
 #define PG_INSTR_CLOCK	CLOCK_REALTIME
 #endif
 
-typedef struct timespec instr_time;
+/* helper for INSTR_TIME_SET_CURRENT */
+static inline instr_time
+pg_clock_gettime_ns(void)
+{
+	struct timespec tmp;
 
-#define INSTR_TIME_IS_ZERO(t)	((t).tv_nsec == 0 && (t).tv_sec == 0)
+	clock_gettime(PG_INSTR_CLOCK, &tmp);
 
-#define INSTR_TIME_SET_ZERO(t)	((t).tv_sec = 0, (t).tv_nsec = 0)
+	return tmp.tv_sec * NS_PER_S + tmp.tv_nsec;
+}
 
-#define INSTR_TIME_SET_CURRENT(t)	((void) clock_gettime(PG_INSTR_CLOCK, &(t)))
+#define INSTR_TIME_SET_CURRENT(t) \
+	((t) = pg_clock_gettime_ns())
 
-#define INSTR_TIME_ADD(x,y) \
-	do { \
-		(x).tv_sec += (y).tv_sec; \
-		(x).tv_nsec += (y).tv_nsec; \
-		/* Normalize */ \
-		while ((x).tv_nsec >= 1000000000) \
-		{ \
-			(x).tv_nsec -= 1000000000; \
-			(x).tv_sec++; \
-		} \
-	} while (0)
+#define INSTR_TIME_GET_NANOSEC(t) \
+	((uint64) (t))
 
-#define INSTR_TIME_SUBTRACT(x,y) \
-	do { \
-		(x).tv_sec -= (y).tv_sec; \
-		(x).tv_nsec -= (y).tv_nsec; \
-		/* Normalize */ \
-		while ((x).tv_nsec < 0) \
-		{ \
-			(x).tv_nsec += 1000000000; \
-			(x).tv_sec--; \
-		} \
-	} while (0)
-
-#define INSTR_TIME_ACCUM_DIFF(x,y,z) \
-	do { \
-		(x).tv_sec += (y).tv_sec - (z).tv_sec; \
-		(x).tv_nsec += (y).tv_nsec - (z).tv_nsec; \
-		/* Normalize after each add to avoid overflow/underflow of tv_nsec */ \
-		while ((x).tv_nsec < 0) \
-		{ \
-			(x).tv_nsec += 1000000000; \
-			(x).tv_sec--; \
-		} \
-		while ((x).tv_nsec >= 1000000000) \
-		{ \
-			(x).tv_nsec -= 1000000000; \
-			(x).tv_sec++; \
-		} \
-	} while (0)
-
-#define INSTR_TIME_GET_DOUBLE(t) \
-	(((double) (t).tv_sec) + ((double) (t).tv_nsec) / 1000000000.0)
-
-#define INSTR_TIME_GET_MILLISEC(t) \
-	(((double) (t).tv_sec * 1000.0) + ((double) (t).tv_nsec) / 1000000.0)
-
-#define INSTR_TIME_GET_MICROSEC(t) \
-	(((uint64) (t).tv_sec * (uint64) 1000000) + (uint64) ((t).tv_nsec / 1000))
 
 #else							/* WIN32 */
 
+
 /* Use QueryPerformanceCounter() */
 
-typedef LARGE_INTEGER instr_time;
+/* helper for INSTR_TIME_SET_CURRENT */
+static inline instr_time
+pg_query_performance_counter(void)
+{
+	LARGE_INTEGER t;
 
-#define INSTR_TIME_IS_ZERO(t)	((t).QuadPart == 0)
+	QueryPerformanceCounter(&t);
 
-#define INSTR_TIME_SET_ZERO(t)	((t).QuadPart = 0)
-
-#define INSTR_TIME_SET_CURRENT(t)	QueryPerformanceCounter(&(t))
-
-#define INSTR_TIME_ADD(x,y) \
-	((x).QuadPart += (y).QuadPart)
-
-#define INSTR_TIME_SUBTRACT(x,y) \
-	((x).QuadPart -= (y).QuadPart)
-
-#define INSTR_TIME_ACCUM_DIFF(x,y,z) \
-	((x).QuadPart += (y).QuadPart - (z).QuadPart)
-
-#define INSTR_TIME_GET_DOUBLE(t) \
-	(((double) (t).QuadPart) / GetTimerFrequency())
-
-#define INSTR_TIME_GET_MILLISEC(t) \
-	(((double) (t).QuadPart * 1000.0) / GetTimerFrequency())
-
-#define INSTR_TIME_GET_MICROSEC(t) \
-	((uint64) (((double) (t).QuadPart * 1000000.0) / GetTimerFrequency()))
+	return t.QuadPart;
+}
 
 static inline double
 GetTimerFrequency(void)
@@ -177,11 +143,42 @@ GetTimerFrequency(void)
 	return (double) f.QuadPart;
 }
 
+#define INSTR_TIME_SET_CURRENT(t) \
+	((t) = pg_query_performance_counter())
+
+#define INSTR_TIME_GET_NANOSEC(t) \
+	((uint64) (((t) * NS_PER_S) / GetTimerFrequency()))
+
 #endif							/* WIN32 */
 
-/* same macro on all platforms */
+
+/*
+ * Common macros
+ */
+
+#define INSTR_TIME_IS_ZERO(t)	((t) == 0)
+
+#define INSTR_TIME_SET_ZERO(t)	((t) = 0)
 
 #define INSTR_TIME_SET_CURRENT_LAZY(t) \
 	(INSTR_TIME_IS_ZERO(t) ? INSTR_TIME_SET_CURRENT(t), true : false)
 
+#define INSTR_TIME_GET_DOUBLE(t) \
+	((double) INSTR_TIME_GET_NANOSEC(t) / NS_PER_S)
+
+#define INSTR_TIME_GET_MILLISEC(t) \
+	((double) INSTR_TIME_GET_NANOSEC(t) / NS_PER_MS)
+
+#define INSTR_TIME_GET_MICROSEC(t) \
+	(INSTR_TIME_GET_NANOSEC(t) / NS_PER_US)
+
+#define INSTR_TIME_ADD(x,y) \
+	((x) += (y))
+
+#define INSTR_TIME_SUBTRACT(x,y) \
+	((x) -= (y))
+
+#define INSTR_TIME_ACCUM_DIFF(x,y,z) \
+	((x) += (y) - (z))
+
 #endif							/* INSTR_TIME_H */
-- 
2.38.0


--ftp5i6iirmkwy7na
Content-Type: text/x-diff; charset=us-ascii
Content-Disposition: attachment;
	filename="v7-0003-instr_time-Add-INSTR_TIME_SET_SECOND.patch"



^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





^ permalink  raw  reply  [nested|flat] 371+ messages in thread

* [PATCH] Lock upgrade without deadlocks.
@ 2026-04-17 13:13 Antonin Houska <ah@cybertec.at>
  0 siblings, 0 replies; 371+ messages in thread

From: Antonin Houska @ 2026-04-17 13:13 UTC (permalink / raw)

For REPACK (CONCURRENTLY), it's essential to upgrade ShareUpdateExclusive lock
to AccessExclusiveLock for the final stage of table processing. However, we
cannot release the earlier before requesting the latter, because if another
process ran DDL on the table in between, REPACK would have to cancel its
transaction, and waste possibly a lot of work. Without releasing
ShareUpdateExclusive lock, the upgrade can cause deadlock, and it's not really
deterministic whether REPACK or the other process will get canceled.

This patch adds a new field to the LOCK structure which tells which process is
trying to upgrade the lock - this field is set right before the upgrade. All
processes waiting in the lock queue are then woken up to check the flag.

When another process tries to get the lock after that, it checks this field,
and if it's set, it checks for deadlocks even if deadlock timeout hasn't
expired yet. If the process is already in the lock's queue and sleeping, the
lock upgrading process wakes it up so it checks the flag immediately.

At the time the upgrading process starts to sleep, all the other process
should be aware that they need to check for deadlock during their lock
acquisitions, so the upgrading process does not have to check for deadlocks
when it gets woken up. Thus it should never receive deadlock error report.
---
 src/backend/commands/repack.c   |   4 +-
 src/backend/storage/lmgr/lmgr.c |  35 ++++++++---
 src/backend/storage/lmgr/lock.c | 107 +++++++++++++++++++++++++++++++-
 src/backend/storage/lmgr/proc.c |  32 +++++++---
 src/include/storage/lmgr.h      |   1 +
 src/include/storage/lock.h      |   5 +-
 src/include/storage/proc.h      |   4 +-
 7 files changed, 166 insertions(+), 22 deletions(-)

diff --git a/src/backend/commands/repack.c b/src/backend/commands/repack.c
index 58e3867246f..310b2a65099 100644
--- a/src/backend/commands/repack.c
+++ b/src/backend/commands/repack.c
@@ -3055,8 +3055,10 @@ rebuild_relation_finish_concurrent(Relation NewHeap, Relation OldHeap,
 	/*
 	 * Acquire AccessExclusiveLock on the table, its TOAST relation (if there
 	 * is one), all its indexes, so that we can swap the files.
+	 *
+	 * TODO The same for indexes and TOAST?
 	 */
-	LockRelationOid(old_table_oid, AccessExclusiveLock);
+	LockRelationOidUpgrade(old_table_oid, AccessExclusiveLock);
 
 	/*
 	 * Lock all indexes now, not only the clustering one: all indexes need to
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 2ccf7237fee..0b103485d54 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -58,6 +58,8 @@ typedef struct XactLockTableWaitInfo
 	const ItemPointerData *ctid;
 } XactLockTableWaitInfo;
 
+static void LockRelationOidCommon(Oid relid, LOCKMODE lockmode,
+								  bool isUpgrade);
 static void XactLockTableWaitErrorCb(void *arg);
 
 /*
@@ -105,6 +107,21 @@ SetLocktagRelationOid(LOCKTAG *tag, Oid relid)
  */
 void
 LockRelationOid(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, false);
+}
+
+/*
+ * Like above, but upgrade an existing lock.
+ */
+void
+LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode)
+{
+	LockRelationOidCommon(relid, lockmode, true);
+}
+
+static void
+LockRelationOidCommon(Oid relid, LOCKMODE lockmode, bool isUpgrade)
 {
 	LOCKTAG		tag;
 	LOCALLOCK  *locallock;
@@ -113,7 +130,7 @@ LockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, isUpgrade);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages, so that we
@@ -157,7 +174,7 @@ ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode)
 	SetLocktagRelationOid(&tag, relid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -191,7 +208,7 @@ LockRelationId(LockRelId *relid, LOCKMODE lockmode)
 	SET_LOCKTAG_RELATION(tag, relid->dbId, relid->relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -254,7 +271,7 @@ LockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, false, true, &locallock,
-							  false);
+							  false, false);
 
 	/*
 	 * Now that we have the lock, check for invalidation messages; see notes
@@ -286,7 +303,7 @@ ConditionalLockRelation(Relation relation, LOCKMODE lockmode)
 						 relation->rd_lockInfo.lockRelId.relId);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -591,7 +608,7 @@ ConditionalLockTuple(Relation relation, const ItemPointerData *tid, LOCKMODE loc
 					  ItemPointerGetOffsetNumber(tid));
 
 	return (LockAcquireExtended(&tag, lockmode, false, true, true, NULL,
-								logLockFailure) != LOCKACQUIRE_NOT_AVAIL);
+								logLockFailure, false) != LOCKACQUIRE_NOT_AVAIL);
 }
 
 /*
@@ -749,7 +766,7 @@ ConditionalXactLockTableWait(TransactionId xid, bool logLockFailure)
 		SET_LOCKTAG_TRANSACTION(tag, xid);
 
 		if (LockAcquireExtended(&tag, ShareLock, false, true, true, NULL,
-								logLockFailure)
+								logLockFailure, false)
 			== LOCKACQUIRE_NOT_AVAIL)
 			return false;
 
@@ -1043,7 +1060,7 @@ ConditionalLockDatabaseObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
@@ -1123,7 +1140,7 @@ ConditionalLockSharedObject(Oid classid, Oid objid, uint16 objsubid,
 					   objsubid);
 
 	res = LockAcquireExtended(&tag, lockmode, false, true, true, &locallock,
-							  false);
+							  false, false);
 
 	if (res == LOCKACQUIRE_NOT_AVAIL)
 		return false;
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index c221fe96889..bcbbf36c091 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -810,7 +810,7 @@ LockAcquire(const LOCKTAG *locktag,
 			bool dontWait)
 {
 	return LockAcquireExtended(locktag, lockmode, sessionLock, dontWait,
-							   true, NULL, false);
+							   true, NULL, false, false);
 }
 
 /*
@@ -829,6 +829,9 @@ LockAcquire(const LOCKTAG *locktag,
  *
  * logLockFailure indicates whether to log details when a lock acquisition
  * fails with dontWait = true.
+ *
+ * isUpgrade should be true if the backend already holds this lock in a lower
+ * mode.
  */
 LockAcquireResult
 LockAcquireExtended(const LOCKTAG *locktag,
@@ -837,7 +840,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 					bool dontWait,
 					bool reportMemoryError,
 					LOCALLOCK **locallockp,
-					bool logLockFailure)
+					bool logLockFailure,
+					bool isUpgrade)
 {
 	LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
 	LockMethod	lockMethodTable;
@@ -1119,7 +1123,8 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		 * case, because JoinWaitQueue() may discover that we can acquire the
 		 * lock immediately after all.
 		 */
-		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait);
+		waitResult = JoinWaitQueue(locallock, lockMethodTable, dontWait,
+								   isUpgrade);
 	}
 
 	if (waitResult == PROC_WAIT_STATUS_ERROR)
@@ -1225,8 +1230,85 @@ LockAcquireExtended(const LOCKTAG *locktag,
 		Assert(!dontWait);
 		PROCLOCK_PRINT("LockAcquire: sleeping on lock", proclock);
 		LOCK_PRINT("LockAcquire: sleeping on lock", lock, lockmode);
+
+		/*
+		 * Lock upgrade can introduce deadlock, therefore enforce special
+		 * behavior of other processes that deal with this lock. In
+		 * particular, any waiter that sees upgradedBy set is expected to
+		 * perform deadlock check as soon as it's woken up. (Since we are
+		 * already in the queue, the deadlock detector has all the information
+		 * it needs.)
+		 */
+		if (isUpgrade)
+		{
+			dlist_iter	iter;
+
+			/*
+			 * There should not be multiple upgrades at the same time. XXX
+			 * ERROR ?
+			 */
+			Assert(lock->upgradedBy == NULL);
+
+			lock->upgradedBy = MyProc;
+
+			/*
+			 * Now, before I sleep myself, wake up all the existing waiters
+			 * (except for me), so they check for deadlock.
+			 */
+			dclist_foreach(iter, &lock->waitProcs)
+			{
+				PGPROC	   *proc = dlist_container(PGPROC, waitLink,
+												   iter.cur);
+
+				if (proc != MyProc)
+					SetLatch(&proc->procLatch);
+			}
+		}
 		LWLockRelease(partitionLock);
 
+		if (!isUpgrade)
+		{
+			HASH_SEQ_STATUS status;
+			LOCALLOCK	*llock;
+			bool	check_deadlock = false;
+
+			/*
+			 * Likewise, new waiters should check the isUpgrade flag and
+			 * engage deadlock detector if needed. The lock being acquired
+			 * should already be in the hash.
+			 *
+			 * Note that, to make deadlock detection happen as soon as
+			 * possible (i.e. regardless deadlock timeout), we check the other
+			 * locks too. XXX There might be ways to get into the deadlock
+			 * indirectly, but that needs more analysis. As long as the
+			 * upgrading process skips deadlock detection altogether, the
+			 * worst consequence of missing some lock wait cycle here is that
+			 * the other process will be kicked-off no sooner than after its
+			 * deadlock timeout has elapsed. (Which in turn means that the
+			 * lock upgrade will take longer than expected.)
+			 */
+			hash_seq_init(&status, LockMethodLocalHash);
+			while ((llock = (LOCALLOCK *) hash_seq_search(&status)) != NULL)
+			{
+				LOCK	*mylock = llock->lock;
+
+				if (mylock && mylock->upgradedBy)
+				{
+					check_deadlock = true;
+					break;
+				}
+			}
+			/* Check for deadlock if needed. */
+			if (check_deadlock)
+			{
+				DeadLockState	deadlock_state;
+
+				deadlock_state = CheckDeadLock();
+				if (deadlock_state == DS_HARD_DEADLOCK)
+					DeadLockReport();
+			}
+		}
+
 		waitResult = WaitOnLock(locallock, owner);
 
 		/*
@@ -1245,6 +1327,18 @@ LockAcquireExtended(const LOCKTAG *locktag,
 			DeadLockReport();
 			/* DeadLockReport() will not return */
 		}
+
+		/*
+		 * If finishing the lock upgrade, we should not get into a deadlock
+		 * anymore, so let others know that they do not have to care either.
+		 */
+		if (isUpgrade)
+		{
+			LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+			Assert(lock->upgradedBy != NULL);
+			lock->upgradedBy = NULL;
+			LWLockRelease(partitionLock);
+		}
 	}
 	else
 		LWLockRelease(partitionLock);
@@ -1322,6 +1416,7 @@ SetupLockInTable(LockMethod lockMethodTable, PGPROC *proc,
 		lock->nGranted = 0;
 		MemSet(lock->requested, 0, sizeof(int) * MAX_LOCKMODES);
 		MemSet(lock->granted, 0, sizeof(int) * MAX_LOCKMODES);
+		lock->upgradedBy = NULL;
 		LOCK_PRINT("LockAcquire: new", lock, lockmode);
 	}
 	else
@@ -1768,6 +1863,12 @@ CleanUpLock(LOCK *lock, PROCLOCK *proclock,
 			elog(PANIC, "proclock table corrupted");
 	}
 
+	/*
+	 * Was this backend upgrading the lock?
+	 */
+	if (lock->upgradedBy == MyProc)
+		lock->upgradedBy = NULL;
+
 	if (lock->nRequested == 0)
 	{
 		/*
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index 1ac25068d62..7b31370db70 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -95,7 +95,6 @@ static volatile sig_atomic_t got_deadlock_timeout;
 static void RemoveProcFromArray(int code, Datum arg);
 static void ProcKill(int code, Datum arg);
 static void AuxiliaryProcKill(int code, Datum arg);
-static DeadLockState CheckDeadLock(void);
 
 
 /*
@@ -1143,7 +1142,8 @@ AuxiliaryPidGetProc(int pid)
  * NOTES: The process queue is now a priority queue for locking.
  */
 ProcWaitStatus
-JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
+JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait,
+			  bool isUpgrade)
 {
 	LOCKMODE	lockmode = locallock->tag.mode;
 	LOCK	   *lock = locallock->lock;
@@ -1230,8 +1230,12 @@ JoinWaitQueue(LOCALLOCK *locallock, LockMethod lockMethodTable, bool dontWait)
 			/* Must he wait for me? */
 			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
 			{
-				/* Must I wait for him ? */
-				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
+				/*
+				 * Must I wait for him? I don't want a deadlock during lock
+				 * upgrade - other processes should fail on it.
+				 */
+				if ((lockMethodTable->conflictTab[lockmode] & proc->heldLocks) &&
+					!isUpgrade)
 				{
 					/*
 					 * Yes, so we have a deadlock.  Easiest way to clean up
@@ -1461,8 +1465,22 @@ ProcSleep(LOCALLOCK *locallock)
 			(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
 							 PG_WAIT_LOCK | locallock->tag.lock.locktag_type);
 			ResetLatch(MyLatch);
-			/* check for deadlocks first, as that's probably log-worthy */
-			if (got_deadlock_timeout)
+			/*
+			 * Check for deadlocks first, as that's probably log-worthy.
+			 *
+			 * Do not wait for the timeout if the lock is being upgraded since
+			 * the risk of deadlock is higher now. However, do not check for
+			 * deadlock if the lock is being upgraded by this process - other
+			 * processes should take care.
+			 *
+			 * TODO Possible optimization: if this is the only lock of the
+			 * backend and if it did not have any weaker lock on the table so
+			 * far, it should be safe to skip the deadlock check. However, to
+			 * evaluate the situation, we need to take fast-path locks into
+			 * account.
+			 */
+			if ((got_deadlock_timeout || lock->upgradedBy) &&
+				lock->upgradedBy != MyProc)
 			{
 				deadlock_state = CheckDeadLock();
 				got_deadlock_timeout = false;
@@ -1819,7 +1837,7 @@ ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock)
  * not, just return.  If we have a real deadlock, remove ourselves from the
  * lock's wait queue.
  */
-static DeadLockState
+DeadLockState
 CheckDeadLock(void)
 {
 	int			i;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 2a985ce5e15..eddbe1e8622 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -38,6 +38,7 @@ extern void RelationInitLockInfo(Relation relation);
 
 /* Lock a relation */
 extern void LockRelationOid(Oid relid, LOCKMODE lockmode);
+extern void LockRelationOidUpgrade(Oid relid, LOCKMODE lockmode);
 extern void LockRelationId(LockRelId *relid, LOCKMODE lockmode);
 extern bool ConditionalLockRelationOid(Oid relid, LOCKMODE lockmode);
 extern void UnlockRelationId(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index ee3cb1dc203..080b69eddfd 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -131,6 +131,7 @@ typedef const LockMethodData *LockMethod;
  * nRequested -- total requested locks of all types.
  * granted -- count of each lock type currently granted on the lock.
  * nGranted -- total granted locks of all types.
+ * upgrader -- process that intends to upgrade the lock
  *
  * Note: these counts count 1 for each backend.  Internally to a backend,
  * there may be multiple grabs on a particular lock, but this is not reflected
@@ -150,6 +151,7 @@ typedef struct LOCK
 	int			nRequested;		/* total of requested[] array */
 	int			granted[MAX_LOCKMODES]; /* counts of granted locks */
 	int			nGranted;		/* total of granted[] array */
+	PGPROC		*upgradedBy;	/* is lock being upgraded by this process? */
 } LOCK;
 
 #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid)
@@ -390,7 +392,8 @@ extern LockAcquireResult LockAcquireExtended(const LOCKTAG *locktag,
 											 bool dontWait,
 											 bool reportMemoryError,
 											 LOCALLOCK **locallockp,
-											 bool logLockFailure);
+											 bool logLockFailure,
+											 bool isUpgrade);
 extern void AbortStrongLockAcquire(void);
 extern void MarkLockClear(LOCALLOCK *locallock);
 extern bool LockRelease(const LOCKTAG *locktag,
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
index 3e1d1fad5f9..c4bc3a24044 100644
--- a/src/include/storage/proc.h
+++ b/src/include/storage/proc.h
@@ -563,10 +563,12 @@ extern bool HaveNFreeProcs(int n, int *nfree);
 extern void ProcReleaseLocks(bool isCommit);
 
 extern ProcWaitStatus JoinWaitQueue(LOCALLOCK *locallock,
-									LockMethod lockMethodTable, bool dontWait);
+									LockMethod lockMethodTable, bool dontWait,
+									bool isUpgrade);
 extern ProcWaitStatus ProcSleep(LOCALLOCK *locallock);
 extern void ProcWakeup(PGPROC *proc, ProcWaitStatus waitStatus);
 extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
+extern DeadLockState CheckDeadLock(void);
 extern void CheckDeadLockAlert(void);
 extern void LockErrorCleanup(void);
 extern void GetLockHoldersAndWaiters(LOCALLOCK *locallock,
-- 
2.47.3


--=-=-=--





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